Rhenium (Re) often exists mainly in molybdenite (MoS2) in nature. Furthermore, molybdenite often coexists in sulfide ores, such as chalcopyrite (CuFeS2), and therefore rhenium has been collected as a by-product of a smelting process of molybdenum or copper.
It has been known that these molybdenite and chalcopyrite, each containing rhenium, are difficult to be perfectly separated from each other by usual ore dressing. Therefore, there has been used a method wherein, in a pyrometallurgy process of recovering copper from chalcopyrite, molybdenite is also simultaneously fed in and melted, and in a gas scrubbing process, rhenium-containing exhaust gas volatilized is collected and rhenium is separated therefrom.
At this time, besides rhenium, a plural kinds of metals, such as arsenic, copper, zinc, and cadmium, coexist in a scrubbing liquid recovered in the gas scrubbing process, in excessive amounts compared with rhenium. As a method for recovering these metal elements from a scrubbing liquid, there has been often used a method wherein a sulfurizing agent, such as gaseous hydrogen sulfide, sodium sulfide, or sodium hydrogen sulfide, is added to the scrubbing liquid to form a precipitate of sulfide. However, in this method, rhenium is also easily precipitated as a sulfide, and therefore rhenium was not able to be selectively recovered.
Accordingly, an operation using an ion exchange method has been performed to selectively separate rhenium from a mixture of a plurality of elements. For example, Patent Literature 1 discloses a method for recovering rhenium, wherein a concentration of sulfuric acid in a liquid for scrubbing sulfurous acid gas generated from a process for smelting nonferrous metals is maintained at not less than 70 g/l, and gaseous hydrogen sulfide is blown into or soluble sulfide is added to the liquid for scrubbing sulfurous acid gas, whereby a sulfide precipitate containing rhenium is formed at an oxidation-reduction potential of 120 to 150 mV (silver/silver-chloride electrode), and then the sulfide precipitate is mixed with a copper sulfate in an acid solution to form a rhenium-containing solution, and the rhenium-containing solution obtained is made to contact a quaternary ammonium salt anion exchange material, whereby rhenium is selectively adsorbed and recovered.
In this method according to Patent Literature 1, when rhenium adsorbed to an anion exchange resin is eluted, an ammonium thiocyanide solution is used. However, there is a possibility that decomposition of ammonium thiocyanide allows formation of toxic cyanide ions. Furthermore, wastewater containing a compound in which ammonium thiocyanide is decomposed has a large environmental load, such as a higher chemical oxygen demand (COD) or a higher nitrogen concentration, and requires higher cost of chemicals necessary for treatment of the wastewater, and thus the method is not advantageous in industrial applications.
Furthermore, a process using an ion exchange method needs a comparatively larger capital investment. Therefore, there was a problem that, depending on a material to be treated, it could be difficult to carry out a design and an operation for increase manufacturing efficiency, such as optimization of equipment capacity.
Furthermore, like in the case of smelting of molybdenum or recycling of a waste catalyst containing rhenium, when a rhenium content in a raw material is comparatively high and rhenium is stably contained therein, a serious problem is not caused, but, when rhenium produced as a by-product of copper smelting is recovered, a rhenium content in a copper ore to be used as a raw material is unstable and greatly varies. Therefore, it is not easy to design and operate equipment to operate efficiently and at a low cost, and also impurity concentration is greatly influenced by a raw material, whereby separation of rhenium is difficult to be carried out stably and highly efficiently.
Therefore, for example, as Patent Literature 2 discloses, there has been recently proposed a process of separating rhenium selectively by using a precipitation separation method in combination.
The process disclosed in Patent Literature 2 is a method capable of separating rhenium stably, efficiently, and at a low cost, even if an amount of rhenium contained in a raw material greatly varies. Specifically, the method is such that rhenium is separated from a solution containing at least any one kind of elements of copper, zinc, cadmium, and arsenic, and perrhenic acid, the method comprising: a first step of adding an alkali, such as sodium hydroxide, to the solution to form a precipitate and performing solid-liquid separation of a solution containing the precipitate; a second step of adding an acid, such as sulfuric acid, to the obtained separated solution to adjust an acid concentration thereof to an equivalent concentration of not less than 1.0 N and not more than 4.0 N; and a third step of adding a sulfurizing agent, such as sodium hydrogensulfide, to the obtained adjusted solution to form a sulfide precipitate, and separating the sulfide precipitate from a post-sulfurization solution.
However, in the case of using the method according to Patent Literature 2, if arsenic is contained in a raw material of rhenium at a high concentration, there has been a problem particularly in a viewpoint of quality control of rhenium products. Arsenic is one of impurities which are difficult to be separated from rhenium, whereas the arsenic content in a rhenium product needs to be controlled to less than 1% by weight, and accordingly the arsenic content of a raw material needs to be reduced. However, arsenic in a raw material is distributed to either a leaching residue resulting after leaching rhenium or a precipitate containing arsenic generated in a purification process. Impurities, such as arsenic, contained in the leaching residue or the precipitate are chemically unstable, and therefore there was no other effective fixing methods but a method of repeatedly making the leaching residue or the precipitate undergo a pyrometallurgy process, and thereby distributing some impurities to stable slag and separating them, and thus the arsenic repeatedly having undergone the pyrometallurgy process was distributed to the raw material of rhenium again, whereby arsenic load continued to be increased.
To fix arsenic in a chemically and environmentally stable form, there is a method wherein arsenic is distributed as slag discharged from a furnace for the above-mentioned pyrometallurgy. In the slag, arsenic is in a glassy state and therefore chemically stable, which is the most preferable. However, in industrial operations, distribution of arsenic to slag is limited in a certain degree and it is not industrially easy to fix a whole amount of arsenic as slag.
On the other hand, as a method of fixing arsenic in a stable form, for example, as Patent Literature 3 discloses, there is a method wherein arsenic is transformed into iron arsenate called scorodite, which has a stable form equivalent to slag. The method according to Patent Literature 3 provides a technique by which, even if some impurity elements exist in an arsenic-containing solution, with treating the solution, there can be synthesized a scorodite compound having high crystallinity and a compact form and resisting swelling caused by moisture or the like, in other words, an iron-arsenic compound excellent in filterability. Specifically, the technique is such that an oxidizing agent is added to a solution having an arsenic concentration of not less than 15 g/L and containing arsenic ions and divalent iron ions, and then, while the solution is stirred, a precipitation reaction of an iron-arsenic compound is allowed to proceed, and the precipitation is terminated when the solution has a pH of 0 to 1.2.
However, Patent Literature 3 does not disclose a method of removing arsenic in a stable form from a raw material, such as a sulfide, containing rhenium and arsenic, while stably maintaining the quality of rhenium. Also, the method according to Patent Literature 3 is a method of leaching out high-concentration arsenic from a sulfide and forming a precipitate of iron (III) arsenate under a low-pH condition, wherein arsenic is imperfectly separated from rhenium and therefore a ratio of arsenic to rhenium (As/Re ratio) in a mother liquor after arsenic separation is higher, and accordingly an arsenic removal load in rhenium purification is very high.
Thus, there has not been proposed an industrially practicable method among a series of the methods wherein arsenic is removed as an impurity in a stable form from a raw material containing rhenium and arsenic, while the quality of rhenium is stably maintained.